A bar code reader which is an optical reading device is disclosed for example in Japanese Utility Publication No. Sho-62-2,689, and the constitution of such a device of the prior art will now be described with reference to FIGS. 10 to 19 of the attached drawings.
FIG. 10 is a sectional view showing the constitution of an optical reading device of the prior art. FIG. 11 is a sectional view of the critical portion of the device of FIG. 10 showing the detecting mechanism. FIG. 12 shows the reading spot of the device of FIG. 11. FIG. 13 is a sectional view of the critical portion of the device of FIG. 11 showing a state in which the detection is made from a separated position. FIG. 14 shows the reading spot of the device of FIG. 13. FIG. 15 is a plan view of the critical portion of the lens 6 of FIG. 10. FIG. 16A is a plan view of bar codes recorded in the media. FIG. 17 is a sectional view showing the state of the bars of FIG. 16. FIG. 17 is a sectional view showing the indicating state of the spaces of FIGS. 16. FIG. 19 is a wave form diagram showing an undesirable detected signals.
A metal body 9 is fixedly installed within a pen-shaped case 1, and one end of the metal body 9 is closed with a base plate 4 which is conductive at its one end. On the inner face of this base plate 4, there are fixedly installed a light emitting element 2 composed of an LED and a light receiving element 3 composed of a photo diode through a die bonding. Terminals 13, 14 which are respectively wire-bonded to the light emitting element 2 and to the light receiving element 3 are projected from the opposite side of the face where the light emitting element 2 and the light receiving element 3 are attached. Thus the base plate 4, the light emitting and receiving elements 2, 3 and the terminals 13, 14 arranged as described above make up a reflective photo sensor 16. This reflective photo sensor 16 is placed as if it is exerting pressure toward the leading end of the pen-shaped case 1 through an insulating member which is made of a high thermal concutivity material such as alumina, and which is installed on the side where the terminals 13, 14 are projected. The other end of the metal body 9 opposite from the side where the base plate 4 is installed is provided with an opening 9a with a predetermined inner diameter, through which light can pass, and a lens 6 made of a transparent plastic member is disposed between the said opening 9a and the light emitting and receiving elements 2, 3. The lens 6 consists of a light emitting side lens 7 and a light receiving side lens 8, the former being for focussing the light emitted by the light emitting element 2 discontinuously to a detection face 10 of the medium 5 in the form of bars 11, while the latter being for focussing to the light receiving element 3 the incoming light reflected from the detection face 10. These light emitting and receiving lenses 7, 8 are fittingly installed within the inside of the metal body 9, and are fixedly positioned by means of a supporting portion 6a. The side of the base plate 4 where the light emitting and receiving elements 2, 3 are located, and which is disposed adjacently to the supporting portion 6a abuts to a metal plate 9b which is in turn attached to the metal body 9 by means of an adhesive layer 9d, thereby fixedly securing the positional relationship between the lens 6 and the light emitting and receiving elements 2, 3.
Both the light emitting side lens 7 and the light receiving side lens 8 are non-spherical lenses, and as shown by the plan view of FIG. 15 where the critical portion of the lens 6 is illustrated, the light emitting side lens 7 and the light receiving side lens 8 are partly overlapping each other, and therefore, a pair of optical axes a, b which are the lines connecting the centres of the refractive curved surfaces of the light emitting side lens 7 and the light receiving side lens 8 will cross each other at a vicinity of the reading spot on the detection face 10 as shown in FIG. 10, in such a manner that the light emitting element 2 is located on the extended line of the optical axis a, and the light receiving element 3 is located on the extended line of the optical axis b.
The beam range transmittable from the light receiving side lens 8 to the light receiving element 3 falls within the angle indicated by the dotted lines b2 and b3 which is the angle of the aperture diaphragm for restricting the light beam, and which is shown in FIG. 10 by the dotted lines b2 and b3, it being based on the flat peripheral portion of the light receiving lens 8 in which a part of the circular shape of the reflected light beam incoming from the detection surface 10 to the light receiving lens 8 is fall out, or biased on the flat shape of the convertible zone (convertible from light to electric signal) of the circular light receiving element. Within the aperture diaphragm formed by the angle between b2 and b3, the angular difference .theta.b between the optical axis b of the light receiving side lens 8 and the maximum angle b2 crossing the optical axis b is determined by the radial distance d between the optical axis of the said lens and the outmost periphery of the lens, the distance 1 between the outmost periphery of the lens and the interesecting point of the optical axis of the lens at the angle of the aperture diaphragm, and the formula 1 as indicated below. EQU Sin.sup.-1 (1/d)=.theta.b (degree) (1)
The numerical aperture (N.A.) as the indicator of the resolving power of the light receiving side lens 8 based on the angle .theta.b is usually set to 0.2-0.4, being determined by the formula 2 as shown below. EQU N.A.=Sin. .theta.b (2)
Meanwhile, the range of concerntrating the beam from the light emitting element 2 by the light emitting side lens 7 falls within the angle of the aperture diaphragm formed by the dotted lines a2 and a3, which is the diaphragm restricting the light beam. Within this angle of the aperture diaphragm between the dotted lines a2 and a3, the angular difference .theta.a between the optical axis a of the light receiving side lens 8 and the maximum angle a2 crossing the optical axis a of the light receiving side lens is determined by the radial distance d between the optical axis a and the outmost periphery of the lens, the distance 1 between the outmost periphery of the lens and the intersecting point of the optical axis of the lens at the angle of the aperture diaphragm, and the formula 3 below. EQU Sin.sup.-1.1/d=.theta.a (degree) (3)
The numerical aperture N.A. as the indicator of the resolving power of the light emitting side lens 8 based on .theta.a is usually set to 0.1-0.4 as determined by the formula 4 below. EQU N.A.=Sin..theta.a (4)
The numerical apertures of the two lenses 7, 8 of this optical reading device are set such that they seem to correspond each other.
The numerical apertures of the two lenses 7, 8 of this optical reading device are set such that they seem to correspond each other.
The narrowing leading end of the pen-shaped case 1, which faces the lens 6, is provided with an aperture 15 of a cylindrical shape for passing the light beams, and through this aperture 15, the adjacently positioned detection surface of the medium 5 sends reflected electrical signal output to the light receiving element 3, the characteristics of the said signal output depending on the light absorbing bars 11 and the light reflecting spaces 12 as shown in FIG. 16. To show the reading positional relationship, the angle between the detection surface 10 of the medium 5 and the axis 0 indicated by a long dotted line which is common to both the reflective photo sensor 16 and the aperture 15 is given 90 degrees in FIG. 10. However, in practical uses, the angle is undulated to 45 degrees (not uniform), and the scanning detection is carried out with such an angle.
The light emitted by the light emitting element 2 is impressed to the reading spot on the detection surface 10 of the medium 5 after being focussed by the light emitting side lens 7. As shown in FIG. 16, since the bars 11 and the spaces 12 are indicated in discontinuous optical codes, when the reading spot is at a bar 11, small amount of reflected light can be obtained, because a large amount of light is absorbed at the bar 11, while, when the reading spot is at a space 12, a large amount of reflected light is obtained because a small amount of light is absorbed at the space 12. A part of this reflected light is focussed by the light receiving side lens 8 to be transmitted to the light receiving element 3 and ultimately to be converted to electric signals.
Thus the bar codes 11, 12 which are provided on the detection surface 10 of the medium 5 are read by the optical reading device, and the high and low voltage levels corresponding to a bar 11 and a space 12 form a pair respectively, these being read by means of a decoder as the time information.
In the conventional optical reading device as described above, if the axis 0 of the reflective photo sensor 16 is directed perpendicularly to the detection surface 10, the output voltages of the space and the bar are reversed each other due to the forward reflections, thereby easily causing reading errors. Within the light beam irradiated onto the detection surface 10 which is perpendicularly disposed to the axis of the photo sensor, a certain light a1 with an angle .theta. is reflected onto the detection surface 10 and at the opposite side around the axis 0, forwardly reflected lights b1 with the same angle .theta. is delivered to the light receiving element 3 proportionately more, thereby making the reading errors be apt to occur.
When reading the optical codes which are the bar codes in the form of the light absorbing bar 11 and the light reflecting space 12 as shown in FIG. 16A, if the above mentioned case is encountered, for example, the detection voltage from the light receiving element for the space 12 will shown always higher output wave form compared with the detection result of the bar 11, as shown in FIG. 16B. But compared with the surface condition of the bar 11 as shown in the enlarged view of FIG. 17, the surface condition of the space 12 as shown in the enlarged view of FIG. 18 shows more unevenness (irregularity) only on the portion where the light absorbing ink is not coated.
Here, within the reflected beam from the detection surface 10 directed perpendicularly to the axis 0, even if a part of the light which is not absorbed but randomly reflected does not reach the light emitting element 3, the proportion of the forwardly reflected light which has the same incident angle and reflecting angle such as light a1 and b1 is increased, and therefore, the wave forms of the detection signals corresponding to the light absorbing bar 11 having a small degree of irregularity and the wave forms of the detection signals corresponding to the light reflecting space 12 will become such that the light absorbing rate at the inked portion comes below such that absorbing rate at the space 12 shown in FIG. 19 in some zone, resulting in that the wave form of the output voltage is disturbed, thereby causing erroneous judgement of the width.
Accordingly, in an optical reading device in which the numerical apertures for the light emitting side lens 7 and the light receiving side lens 8 are approximately same each other, the proportion of the forwardly reflected light amount having the reflection angle .theta. and reaching the light receiving element 3 to the light amount irradiated to the reading spot X with an incident angle .theta. becomes large. Therefore, particularly, due to the degree of the surface unevenness of the light absorbing face or the medium, and due to the variations of the direction of the optical reading device, the detection output from the light reflection portion and the light absorbing portion partially reverses each other, thereby increasing the probability of causing judgement errors.
As a means for preventing the errors due to this forward reflection, the complementation of the non-spherical surface of the light emitting side lens 7 is made imperfectly. For example, it may be conceived that, with a short distance between the lens and the light source, the spherical aberration due to the difference between the distance from the centre of the lens to the light source and the distance from the periphery of the lens to the light source is made large, thereby reducing the component of the passing light by making it reflected to the reading spot X. But in this case, the light from the light emitting element 2 equivocates the irradiated spot at the reading spot X, and therefore, the desired amount of reflected light can not be obtained, resulting in that the detectable resolving function of the optical reading device is lowered.
On the other hand, if the distances of the detection surface 10 of the medium 5 and the optical reading device are increased from the positions of FIG. 10 and 11 to the position of FIG. 13, the irradiated spot .alpha. of the light from the light emitting element 2 deviates from the detection spot .beta. on the detection surface 10 as shown in FIG. 14, unlike that of FIG. 12. This phenomenon is mainly due to the fact that the optical axis a of the light emitting side lens 7 and the optical axis b of the light receiving side lens 8 are made to intersect at the reading spot near the aperture 15, and the light emitting and receiving elements 2, 3 are disposed on the said optical axes respectively, resulting in that the comatic aberration which is a non-objective deviation of the image formed at the focal spot is extremely reduced. In an optical reading device in which this comatic aberration is inhibited, if the detection surface 10 is separated away from the reading spot which is also the intersecting point of the optical axes a, b, the irradiated spot .alpha. is deviated from the detection zone .beta. as well as the illumination within the irradiated spot being lowered, thereby drastically decreasing the light-electro conversion within the light receiving element 3 and ultimately lowering the detection capability. If the comatic aberration or the spherical aberration is further inhibited, the irradiated spot .alpha. is properly positioned at the reading spot X as shown in FIG. 12, but as the irradiated spot is gradually deviated from the reading spot X as shown in FIG. 14, the illumination is gradually weakened although the concentration is still kept, bringing the effect that a larger illumination is made, thereby lowering the resolving power for the minimum readable interval of the light absorbing pattern and the light reflecting pattern on the detection surface 10 of the medium 5. Thus in an optical reading device, only when more than a set value of a focal depth W (for example, about 1.2 mm) which is the readable limit is secured after deviation from the reading spot X, the operation of detection is assured.
But the conventional optical reading devices are constituted such that the same light emitting and light receiving side lens axes are made to intersect each other at a vicinity of the reading spot, and the light emitting and light receiving elements are disposed on the said optical axes respectively, resulting in that, if the irradiated spot .alpha. is equivocated in order to prevent reading errors due to forward reflections, the resolving power is lowered to ultimately lower the reading probability.